Several different types of television broadcast formats and standards are used in different locals throughout the world. Two of the most pervasive are the system used in the United States of America known as NTSC (National Television Standards Committee) and the European system known as
(Phase Alternating Line). In addition, several systems for providing increased picture resolution or definition, generally referred to as HDTV (High Definition Television) have been and are continuing to be developed.
While the television receivers operating in these varied systems are equally varied, certain aspects remain generally similar. For example, most television receivers include circuitry for selecting the desired television signal from among a plurality of broadcast signals available, a signal processing system which recovers the picture and sound information from the broadcast signal, systems for sequentially scanning a display device such as a cathode ray tube in both horizontal and vertical directions, and scan synchronization systems operative upon the display to coordinate display scanning to the picture and sound information.
Despite significant differences between the signal selection and signal processing functions of television receivers operating in accordance with the above-mentioned variety of broadcast systems, the functions of display scanning and synchronization are generally quite similar. Generally, picture and sound information together with scan synchronizing signals are modulated upon a broadcast carrier at the transmitter. At the receiver, the scan synchronizing signals are separated from the remainder of the picture and sound information and used to control locally generated horizontal and vertical scan signals. The latter are used to drive the scanning circuits of the display system.
Computer monitors and many video game devices are also similar to television receivers in that they include a display system, such as a cathode ray tube, which is scanned in synchronism with picture information. In such systems, the scan signals are computer generated and are used to synchronize display scan and picture information in much the same manner as television receivers.
Because the horizontal scan oscillator control used in the variety of television receivers, computer monitors and video game displays are substantially identical in overall function, it seems logical to provide a single system which may be operated for all such uses (e.g. a "universal" system). However, attempts to do so have been frustrated largely by the broad range of scan frequencies over which such a universal scan control system would be required to operate.
As is well known, the operation of display system synchronization controlling the horizontal scan oscillator is extremely demanding. Meeting these demands is made challenging by the need for precise control of both the frequency and phase of the horizontal scan oscillator. Maintaining frequency control over a broad range of reference scan frequencies generally leads to systems which lack stable precise phase control. Conversely, highly precise stable phase locking systems usually lack the ability to accommodate a wide range of frequencies.
For example, one of the most common and pervasive horizontal scan oscillator control systems used in television receivers, computer monitors, or video games is generally referred to as a phase locked loop. While a variety of different phase locked loop systems have been developed, virtually all use a phase detector which compares an oscillator output signal to the received reference synchronizing signals to produce an error voltage indicative of the difference therebetween. This error voltage is filtered by a low pass filter and amplified by an error amplifier to produce a controlling voltage for the voltage controlled oscillator. In the event the voltage controlled oscillator is precisely synchronized to the referenced synchronizing signals, the output voltage of the phase detector is zero and the frequency of the voltage controlled oscillator remains unchanged. In practice, however, this condition seldom exists and, more typically, the frequency of the voltage controlled oscillator residually differs from that of the incoming reference signals. If the difference between the oscillator and the reference signals detected by the phase detector is a phase difference or minor frequency difference, the error voltage coupled to the oscillator is sufficient to cause the oscillator to change frequency and bring its output signal toward synchronization with the reference sync signals. It has been found that most prior art phase locked loop systems respond well enough to minor variations of oscillator phase and frequency.
In most oscillator control systems including phase locked loop systems and the like, the system characteristics utilized during the synchronizing or lock-in operation of the system differ substantially from the characteristics desired during synchronization once the frequency lock has been obtained. In addition, systems having increased sophistication and complexity such as that set forth in the above-related copending application may function differently in certain subtle aspects as the system moves between conditions of acquisition of phase and frequency lock, short term lock and synchronization, and/or the existence of long term undisturbed frequency and phase lock. Accordingly, there arises a need for oscillator control systems which may perform these subtle adjustments and thereby optimize system performance under a variety of operating conditions.
Accordingly, it is general object of the present invention to provide an improved horizontal scan oscillator control system operable in response to a broad range of scan frequencies. It is a more particular object of the present invention to provide an improved horizontal oscillator control system which responds to broad frequency signals while optimizing system performance during frequency synchronization acquisition, short term phase and frequency synchronization, and long term phase and frequency synchronization.